The invention relates to cured rubber compositions having high modulii at low strain. More particularly, the invention relates to cured rubber compositions having modulii at 10% elongation of from about 50 MPa to about 200 MPa which are formed by curing, under specified temperature and time conditions, curable rubber compositions comprising a rubber selected from natural rubber, nitrile rubber, neoprene and blends thereof; a metal dimethacrylate selected from zinc dimethacrylate and magnesium dimethacrylate and a peroxide curing agent and optionally, a co-curing agent.
Conventional cured rubber compositions normally exhibit excellent flexibility and extensibility having elongations at break of 200% or greater. Unfortunately, however, such compositions also normally exhibit low modulus values at low strains. Thus, such compositions typically have modulus values at 10% elongation of less than about 10 MPa, more commonly about 5 MPa or less. On the other hand, cured polymeric compositions which exhibit high modulus values at 10% elongation normally have low elongations at break and are therefore brittle like plastics.
Cured rubber compositions having high modulii at low strain and good extensibility would have significant advantages in certain applications such as, for example, tire compounds. Accordingly, a need for such compositions exists.
Curable rubber compositions containing various rubbery polymers, metal acrylates or metal methacrylates including basic zinc methacrylate and zinc dimethacrylate and peroxide curing agents and certain cured products formed from such compositions are known in the art as illustrated by the following patents.
U.S. Pat. No. 4,056,269 to Pollitt et al. relates to molding compositions suitable for forming molded golf balls containing cis-polybutadiene or other undefined elastomers, a metal-containing crosslinking monomer and a peroxide initiator. A wide variety of metal-containing crosslinking monomers are disclosed in columns 2 and 3 of the patent including zinc diacrylate, zinc dimethacrylate and magnesium dimethacrylate. The patent, at column 4 lines 3-14, teaches that the amount of the metal-containing crosslinking monomer should correspond to at least about 0.046 equivalents of polymerizable unsaturation per mole of butadiene in the elastomer base, but may be as high as 0.41 equivalents per mole, which when converted to a parts by weight basis indicates a range of from about 9 to 90 parts by weight of the crosslinking monomer per 100 parts by weight of cis-polybutadiene. The reference further teaches that if the crosslinking monomer selected is zinc dimethacrylate, the more preferable amounts are in the range of about 20 to 50 parts per 100 parts of elastomer. The peroxide initiator may be present in amounts of about 0.2-10% by weight of the elastomer. The reference additionally discloses that molding temperatures may range between 130.degree. and 200.degree. C. and curing times may range from 1 to 50 minutes.
U.S. Pat. No. 4,065,537 to Miller et al. relates to a process for producing molded golf balls exhibiting isometric compression which involves the use of cylindrical slugs about 2.5 to 3.3 inches long having substantially flat or convex top and bottom surfaces during the molding process. Molding compositions for forming such slugs are disclosed which contain cis-polybutadiene or other elastomers, a metal-containing crosslinking monomer and a peroxide initiator. A wide variety of metal-containing crosslinking monomers are disclosed including zinc dimethacrylate and magnesium dimethacrylate. The compositions disclosed are basically the same as those described in U.S. Pat. No. 4,056,269 above with the exception that the preferred amounts of metal-containing crosslinking monomer, especially when said monomer is zinc dimethacrylate, are stated as ranging from about 15 to 60 parts per 100 parts of cis-polybutadiene. Curing conditions disclosed are the same as set forth in the aforementioned patent.
U.S. Pat. No. 4,082,288 to Martin et al. discloses free-radical crosslinkable compositions containing a peroxide crosslinkable elastomer, from about 10 to about 60 parts by weight per 100 parts by weight of elastomer of basic zinc methacrylate and a peroxide curing agent. The examples of the patent indicate that the compositions are cured at 300.degree. F. (148.9.degree. C.) for 20 minutes.
U.S. Pat. No. 4,191,671 to Kataoka et al. relates to abrasion resistant rubber compositions which are prepared by curing rubber compositions comprising: (A) a diene elastomer, (B) an alpha-beta ethylenically unsaturated carboxylic acid, wherein the ratio by weight of component (A) to component (B) is 87/13 to 55/45, (C) a divalent metal compound being present in quantities of 50 to 150 parts by weight per 100 parts by weight of component (B), and (D) an organic peroxide being present in quantities of 0.3 to 5.0 parts by weight per 100 parts by weight of the combined weight of components (A) and (B). The compositions may additionally contain an unpolymerizable carboxylic acid, carbon black in amounts of less than 50 parts by weight per 100 parts by weight of elastomer and an amine and/or phenol compound. The patent broadly discloses curing temperatures ranging from 110.degree.-180.degree. C. while the examples thereof exemplify curing temperatures ranging from 120.degree. to 140.degree. C. and curing times ranging from 30 minutes to 60 minutes.
U.S. Pat. No. 4,264,075 to Miller et al., relates to a two-piece molded golf ball consisting of a cover comprising an ionic copolymer and a center part formed from a molding composition containing cis-polybutadiene or other elastomers, a metal-containing crosslinking monomer and a peroxide initiator. A wide variety of such crosslinking monomers are disclosed including zinc dimethacrylate and magnesium dimethacrylate. The molding compositions used to form the center part of the golf ball are substantially the same as those described in U.S. Pat. Nos. 4,056,269 and 4,065,537 above with the exception that lower quantities of crosslinking monomer are preferred, i.e. from 15 to 35 parts per 100 parts of cis-polybutadiene. Molding temperatures disclosed in the patent may range between 140.degree. C. and 200.degree. C., advantageously about 150.degree. to 190.degree. C., preferably 170.degree. to 185.degree. C. and curing times may range from about 50 to 1 minutes, advantageously from 30 to 5 minutes and preferably about 20 to 10 minutes.
U.S. Pat. No. 4,266,772 to Martin et al. relates to solid golf balls formed from a curable elastomer composition comprising a free radical crosslinkable elastomer, especially a peroxide crosslinkable elastomer; from about 10 to about 60 parts by weight of a zinc oxide-methacrylic acid reaction product, preferably basic zinc methacrylate, per 100 parts by weight of elastomer and a curing agent such as a peroxide curing agent. Curing conditions set forth in the examples are 20 minutes at 300.degree. F. (148.9.degree. C.).
U.S. Pat. No. 4,305,851 to Tominaga et al. relates to a process for preparing a solid golf ball which comprises incorporating a microencapsulated zinc salt of an allyl group containing carboxylic acid as a crosslinking agent into a rubber along with a peroxide curing agent and other additives to produce a rubber composition, following which the rubber composition is molded. The patent at column 5, Table I discloses several polybutadiene rubber compounds containing microencapsulated zinc dimethacrylate and peroxide curing agents with the zinc dimethacrylate being present in amounts of 42 and 50 parts respectively. The compositions are cured for 30 minutes at 150.degree. C.
U.S. Pat. No. 4,500,466 to Hayes et al. relates to a method of preparing a zinc dimethacrylate adjuvant of high surface area and to vulcanizable polymeric compositions containing the adjuvant. Vulcanizable polymeric compositions which are disclosed comprise: (a) rubbery polymers selected from the group consisting of natural rubber, ethylene/propylene copolymers, ethylene/propylene/diene terpolymers, styrene/butadiene copolymers, nitrile rubbers, neoprene and blends thereof; (b) from about 25 to about 85 parts by weight of a zinc dimethacrylate powder having a surface area of from about 3.7 to about 5.4 m.sup.2 /g or more per 100 parts by weight of said rubbery polymers; and (c) a cure effective amount of a peroxide curing agent. The specification discloses that the amount of peroxide generally used may range from about 0.2 to about 2.0 parts by weight. Example 20 of the patent discloses a composition containing 100 parts of natural rubber, 60 parts of zinc dimethacrylate and 1.0 parts of Vulcup R, i.e., bis-(t-butylperoxy) diisopropyl benezene. As shown in Table IV, this composition when cured for 15 minutes at 160.degree. C. exhibits a 10% modulus of 3.2 MPa. Example 21 discloses a composition containing 100 parts of natural rubber, 70 parts of zinc dimethacrylate and 1.0 parts of said peroxide curing agent. This composition when cured for 15 minutes at 160.degree. C. exhibits a 10% modulus of 4.6 MPa.